BlackCorvette

Thanks, I will set it up as you described..

These relay boards nearly always use an opto-isolator to control the relay, they can work safely with a Teensy 4.x only if they have a separate logic VCC instead of connecting the opto-isolators directly to the same voltage supply as the relays.

You won't be using Teensy pins in input mode with that relay board, so 5V tolerance of input mode isn't a factor. What is a factor is whether that relay board can work with the 3.3V output signals Teensy transmits. It's difficult to give a 100%...

Thanks. I believe the 3.3v output is where the problem lies since this is a 5v module.

(4 relay module) Is the Teensy 4.0 able to control one of these Auduino 4 relay modules ?